Fuel cells are electrochemical devices that produce direct current (DC) electricity by the reaction of a fuel with an oxidant, typically producing byproducts of heat and water. Common fuels are hydrogen, methanol, and carbon monoxide; however, carbon monoxide can only be used as a fuel in high-temperature fuel cells operating at temperatures greater than 400° C. The most common oxidant is oxygen, either in a relatively pure form or from ambient air. Fuel cells contain an anode, a cathode, and an electrolyte barrier between the anode and cathode. The fuel is introduced at the anode and the oxidant is introduced at the cathode. The electrolyte barrier, commonly referred to as a membrane-electrode assembly or MEA, is an ionically conductive thin barrier that is relatively impermeable to the fuel and oxidant, and is electrically insulating. Known fuel cell designs and operating principles are described in, for example, The Fuel Cell Handbook, 7th Edition (2004) published by the US Department of Energy, EG&G Technical Services under contract DE-AM26-99FT40575 and in co-owned U.S. patent application Ser. No. 11/484,514, filed Jul. 6, 2007; and Ser. No. 12/006,893, filed Jan. 7, 2008, each of which is incorporated herein by reference.
Many configurations of fuel cell systems are known. Portable fuel cell systems are based on several different types of fuel cells, including proton-exchange membrane fuel cells (PEMFC) that operate at temperatures less than 85° C. and that use high-purity hydrogen as the fuel; PEMFCs that operate at temperatures in the 135° C. to 200° C. range and that use hydrogen-rich reformate as the fuel; direct methanol fuel cells (DMFC) that operate at temperatures less than 85° C. and that use methanol as the fuel; and solid oxide fuel cells (SOFC) that operate at temperatures in the range of 500° C. to 900° C. and that use hydrogen-rich reformate as the fuel.
Fuel processors prepare the fuel supply for use by the fuel cell. Often the fuel processor has many components including a vaporizer or reformer. Conventional reformers are a bundle of tubes having large diameters in the range of 25-150 mm. Each tube is packed with granules or bulk material to form a catalytic bed. Such tubes are relatively inexpensive and the technology has been utilized to meet large scale requirements. Mechanical events such as vibrations and shocks can break down the bed. Often, channels form that undesirably create flowpaths that allow the fuel stream to pass without significant reaction.
The fuel preparation process is also endothermic so that heaters are used to externally apply heat to the tubes to increase process efficiency. Due to the large size and wall thickness of the tubes, the reaction to the heating process is relatively slow (i.e., an undesirable gradient occurs). Further, the bed can break down during this thermal cycling.
Velocys, Inc. of Plain City, Ohio has developed an alternative microchannel reactor in an effort to overcome the slow heat gradient. For example, see U.S. Pat. Nos. 7,250,151; 7,029,647; 7,014,835; and 6,989,134, each of which is incorporated herein by reference. Velocys, Inc. forms microchannels of 0.1-1.0 mm in a thin metal plate. Because the microchannels are so small, a bulk material cannot be used as a catalyst. Rather, a wash coat of a catalyst material is applied. Hence, the heat applied to the plate is very quickly transferred to the reaction zone. To scale up the microchannel technology, a plurality of plates are stacked. Unfortunately, the microchannel technology is expensive to manufacture and heavy as a large amount of a metal such as steel is necessary.
Thus, there is a need for a fuel processor for a fuel cell system that is affordable, has a small temperature gradient and is robust under mechanical duress and thermal cycling. The present invention addresses these needs among others.